The present invention relates to a drop-out recording system of a video recording magnetic tape, in particular, it relates to such a system which records the voice channel signal on a voice channel of said magnetic tape depending upon the drop-out in a video channel. The present drop-out recording system can be utilized for the laboratory analysis of a drop-out of a magnetic tape in order to provide an improved magnetic tape.
A drop-out or a defect on a video recording magnetic tape deteriorates the picture quality of a reproduced image, therefore, we must detect the drop-out on a magnetic tape, and analyze the reason why said drop-out occurred on a magnetic tape in order to provide the improved picture quality and/or a drop-out free magnetic tape.
A drop-out of a video recording magnetic tape is defined by having the signal level of the particular portion on the tape lower than the normal level for instance by more than 18 dB, and the duration of that low level is more than one half of the scanning time. Since the scanning time of each scanning line in a present television standard system is approximate 63 .mu.S, said duration corresponds to approximate 20 .mu.m of length when the relative speed of a tape to a magnetic head is 6 meter/second. According to some specifications for the purchasing of a video recording magnetic tape, the number of drop-outs in one minute must be less than 30.
A drop-out on a magnetic tape results from dust on a tape, the injury of the plastic base material, and/or a lump of magnetic material on a tape. By analyzing the drop-outs statistically, the information for improving a magnetic tape in the manufacturing process is obtained.
It should be noted that there are many defects on a magnetic tape, some are big defects that give large deterioration of the picture quality, and some are small defects which give little deterioration of a picture quality. Therefore, for laboratory analysis purposes, a defect which gives the substantial deterioration of the picture quality having the level less than -18 dB, and the duration longer than one half of a scanning line must be selected from all the defects, and that substantial defect must be analyzed for improving the quality of a magnetic tape in the manufacturing process of said tape.
According to one of the prior drop-out detecting systems, a magnetic tape to be tested is developed magnetically by utilizing the particular reagent including ferro-magnetic material which is for instance powdered carbonyl iron contained in a liquid. After developing a magnetic tape magnetically, powder of ferro-magnetic material is attached on the surface of a magnetic tape, but said powder does not attach on the drop-out portion. Accordingly, a drop-out can be detected visually. However, this chemical method for detecting a drop-out has the disadvantage that it does not enable us to select the substantial drop-out which gives the level lower than -18 dB and the duration longer than one half of a scanning line. This is to say, although we can recognize a drop-out visually, we can not determine the grade of the picture deterioration by the recognized drop-out.
According to another prior drop-out detecting system, a drop-out is detected electrically by running the magnetic tape to be testd along a magnetic head, which reads the electrical signal recorded on the tape. If the reproduced electrical signal is interrupted for a predetermined duration, the drop-out is then recognized. However, this electrical system of drop-out detection has the disadvantage that we can not determine which position on a magnetic tape provides the electrically detected drop-out.
According to one proposal for overcoming the disadvantages of the prior arts, the magnetic tape to be tested runs along a magnetic head, and when a drop-out is electrically detected, an erasing current is provided to a magnetic head which erases the records on a tape just after a drop-out. Thus, a large artificial drop-out is provided just after the actual drop-out. Next, by developing magnetically the tape using a reagent, both the artificial drop-out and the actual drop-out appear visually. The actual drop-out is recognized since it is just after the large artificial drop-out. However, that proposal has the disadvantage that a strong erasing current must be provided on a magnetic head during the read operation of said magnetic head. Therefore, one must change some circuit around a magnetic head of a video tape recorder just for the test purpose for a drop-out. This change accounts for a high cost of testing.